Demà he d'explicar la resposta a la professora abans de la classe.

Catalan often puts a time expression at the beginning of the sentence to set the scene, just like English can do with Tomorrow, I have to....

So:

• Demà he d'explicar... = Tomorrow I have to explain...

This does not change the basic meaning; it just makes tomorrow the first thing the listener hears.

You could also say:

• He d'explicar la resposta a la professora demà...

but Demà at the front sounds very natural.

It is built into the verb he.

In Catalan, subject pronouns are often omitted because the verb form already tells you who the subject is.

• he = I have
• so he d'explicar = I have to explain

You could say Jo he d'explicar..., but jo is usually unnecessary unless you want emphasis, contrast, or clarity.

He de + infinitive is a common Catalan way to express obligation, similar to have to in English.

So:

• he d'explicar = I have to explain

The full pattern is:

• haver de + infinitive

Examples:

• He d'estudiar = I have to study
• Has de venir = You have to come
• Han de marxar = They have to leave

In your sentence, he de explicar becomes he d'explicar because of elision.

This is because de is shortened before a vowel.

• de + explicar → d'explicar

Catalan uses apostrophes a lot to avoid awkward vowel sequences.

So:

• he d'explicar
• has d'anar
• hem d'obrir

This is completely normal and required in standard writing.

It is the same verb form, but it is being used in a different structure.

• he menjat = I have eaten
  Here he is an auxiliary used to form the perfect tense.

• he d'explicar = I have to explain
  Here he is part of the obligation structure haver de + infinitive.

So the word he is the same form, but the meaning depends on what comes after it:

• he + past participle → perfect tense
• he de + infinitive → obligation

They do different jobs.

• la resposta: here la is the definite article = the answer
• a la professora: here la is also the definite article, but it comes after the preposition a, so together a la = to the teacher

So:

• la resposta = the answer
• a la professora = to the teacher

Even though they look similar, one is just an article by itself, and the other is a preposition + article combination.

Because la professora is the person receiving the explanation, so it functions as an indirect object: to the teacher.

In Catalan, that is introduced with a:

• explicar alguna cosa a algú = to explain something to someone

So:

• la resposta = the thing being explained
• a la professora = the person it is explained to

This is very common in Catalan.

Yes.

Instead of a la professora, you could say:

• Demà li he d'explicar la resposta abans de la classe.

Here:

• li = to her / to him

You can also combine both in some contexts for emphasis or clarity:

• Demà li he d'explicar la resposta a la professora abans de la classe.

This kind of doubling is common in Catalan, especially with people.

Because abans normally takes the preposition de before a noun or another element.

So:

• abans de la classe = before class
• abans de dinar = before lunch
• abans de sortir = before leaving

Think of abans de as a fixed pattern meaning before.

Yes. Here la classe means the class / the lesson.

So:

• abans de la classe = before class / before the lesson

Depending on context, classe can refer to:

• a lesson
• a school class
• a classroom session

In this sentence, it most naturally means the class that will happen later.

Yes, in some contexts that is possible.

• abans de classe = before class, in a more general sense
• abans de la classe = before the class, referring to a specific class

With la, it sounds more specific: a particular class is meant.

So your sentence suggests a specific class known to the speakers.

Both orders are possible.

• explicar la resposta a la professora
• explicar a la professora la resposta

The version in your sentence is very natural and clear:

1. the thing explained
2. the person receiving it

Catalan word order is somewhat flexible, but this order is common and easy to process.

Because demà works directly as an adverb meaning tomorrow.

So Catalan simply says:

• Demà = tomorrow

No preposition is needed, just as in English you say tomorrow, not on tomorrow.

Yes.

• professora = female teacher
• professor = male teacher

The article also matches:

• la professora
• el professor

So this sentence specifically refers to a woman teacher.

You could keep a la professora in a stronger position or add the pronoun li:

• Demà li he d'explicar la resposta a la professora abans de la classe.

This can give extra prominence to the teacher.

You could also stress it in speech. Catalan often uses pronouns plus the full noun phrase when the speaker wants emphasis or clarity.

The h in Catalan is silent.

So he is pronounced roughly like eh.

And because of the apostrophe:

• he d'explicar

flows together smoothly.

Very roughly:

• he ≈ eh
• d'explicar begins with the explicar sound, with the shortened d' attached before it

So the h is not pronounced at all.